tty: serial: fsl_lpuart: flush RX and TX FIFO when lpuart shutdown

Need to flush UART RX and TX FIFO when lpuart is shutting down to make
sure restore a clean data transfer environment.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20250107074834.3115230-1-sherry.sun@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>